1. Field of the Invention
The present invention relates to a polarizing apparatus that acts both as a polarizer and an analyzer and also to a projection display apparatus that enlarges and projects optical images formed on a reflection type light valve using the above-mentioned polarizing apparatus.
2. Description of the Prior Art
In order to obtain large-size picture images, a hitherto known method is to form optical images on a light valve operating by video signals, illuminate a projection light on this optical images and project the optical images in enlarged size on a screen through a projection lens. In recent years, a projection display apparatus using a liquid crystal display as a light valve attracts much attention. And upon aiming at the high resolution capability, the Japanese unexamined Patent publication (Tokkai Sho) 61-13885t (M. Himuro) proposed a reflection type liquid crystal display that is easily expandable to a large capacity apparatus with regard to the pixel number without spoiling the pixel aperture factor of a liquid crystal display.
An outline configuration of a conventional projection display apparatus using the above-mentioned liquid crystal display is shown in FIG. 19. A light beam 2 emitted from a light source 1 is collimated into a nearly parallel beam, and then the light beam is separated into an S-polarization component 4 which is reflected by a polarization beam splitter 3 and a P-polarization component 5 which transmits therethrough. The S-polarization component 4 is incident upon a liquid crystal display 6. The liquid crystal display 6 is of the kind that utilizes a property of the birefringence of a liquid crystal, and each picture element has individual reflecting electrode for reflecting light. In case that no voltage is applied to the liquid crystal layer, no birefringence property is exhibited actually, whereas when voltage is applied, the birefringence property takes place. Therefore, when a linearly polarized light having its polarization direction in a specified direction is incident, its reflected light becomes an elliptically polarized light.
The S-polarization component 4 is partially converted into the P-polarization component by the liquid crystal display 6 and is incident again into the polarization beam splitter 3. The P-polarization component included in the reflected light from the liquid crystal display 6 transmits through the polarization beam splitter 3 and incident into a projection lens 7, whereas the S-polarization component is reflected thereby and proceeds toward the light source 1. In such the manner, an optical image formed as the change of the birefringence property on the liquid crystal display 6 is expanded and projected onto a screen (not shown) by a projection lens 7.
In the reflection type liquid crystal display device, the switching elements can be disposed behind the picture element electrodes. Therefore, the picture element pitch can be reduced without reducing the size of the switching elements, and hence the high density integration of the picture element is not a difficult task. Therefore, by the use of the reflection type liquid crystal display device, a brighter and higher resolution projection image in comparison with the use of the transmitting type liquid crystal display device can be obtained.
In the configuration shown in FIG. 19, when black is to be displayed, the S-polarization component 4 incident onto the liquid crystal display 6 is reflected back as it was (S-polarization component) without receiving any conversion into the P-polarization component. The more the amount of residual S-polarization component transmitting through the polarization beam splitter 3 and being incident onto the projection lens 7, the less the resultant contrast of the projection image becomes. Therefore, in order to obtain a high contrast projection image, it is required to make the S-polarization component transmittance of the polarization beam splitter 3 very low.
In general, as for the polarization beam splitter 3, those type as that proposed by S. M. MacNEILLE in the U.S. Pat. No. 2,403,731 are mostly used. The U.S. Patent discloses an optical device in which two glass prisms are joined together to form a cubic shape or a rectangular parallelo-piped shape, and therein an optical multi-layers thin film is formed on the joining surfaces. The optical multi-layers thin film is formed by alternately laminating two different kinds of thin films each of which has a mutually different refractive index from the others. This configuration has an action of separating the natural light into two polarization components having mutually-orthogonal planes of polarization. Materials and the film thicknesses of those two thin films are selected to fulfill the Brewster's angle condition that the transmittance of the P-polarization component becomes 100% at a specified wavelength. This Brewster's angle condition is expressed with the incident angle .theta..sub.G onto the multi-layers thin film, the refractive index n.sub.G of the glass prisms, the refractive index n.sub.L of the lower refractive index thin film, and the refractive index n.sub.H of the higher refractive index thin film, as follows: ##EQU1##
When the condition of EQ.(1) is fulfilled, it is possible, with keeping the transmittance of the P-polarization component to 100%, to reduce the transmittance of the S-polarization component by increasing the number of layers of the multi-layers thin film.
However, the wavelength bandwidth of the polarization beam splitter wherein separation of the two polarization components are carried out is relatively narrow. Therefore, in case of a projection display apparatus using white light, it is difficult to keep the transmittance of the S-polarization component low over the whole wavelength bandwidth over the visible light range.
For this problem there has been such a proposal that the white, light emitted from a light source is decomposed into three primary colors of red, green, and blue, and three polarization beam splitters respectively corresponding to those three wavelength band ranges are provided.
According to such conventional configuration, even though transmittances of those respective S-polarization components at respective wavelength ranges of particular color components can be kept low, cost naturally rises and also the size of apparatus and weight increase since three of joined block of glass prisms are used. Moreover, since it is usually the case that the incident light is not perfectly parallel, in such a case, wavelength shift is liable to take place owing to an incident angle dependence of the light. And a narrow wavelength bandwidth of the transmittance of the S-polarization component becomes practically further narrower. Accordingly, when it is intended to obtain a high contrast projection image with such configuration, the substantially parallel light only must be used. Therefore, a difficulty arises in producing sufficient bright picture images.